The present invention relates to a screen structure as defined in the preamble of claim 1 for use in the manufacture of a fiber product.
The screen structure of the invention is applicable for use directly as such or with minor changes in different stages of the production process of a fiber product, such as paper. The screen structure of the invention is particularly well suited for use in dry formation of a web material. In the dry formation process referred to, a distribution unit, i.e. a so-called former is used, which generally has an even number of screen tubes. The screen tube of the former is typically a cylindrical tube whose envelope surface is e.g. a substantially thin perforated plate bent into a cylindrical form, the fiber material being spread by suction through these perforations onto a forming wire below the screen tube in a direction transverse to the direction of motion of the forming wire. The fiber layer formed on the forming wire can be made more uniform by using an even number of transverse screen tubes and blowing a fiber flow in opposite directions in alternate screen tubes.
The screen structure of the invention is also applicable for use in a hammer mill, which is generally placed before the former and in which the fiber web is reduced to fibers of suitable size before the fiber flow is passed to the former. In this case, the cylindrical envelope of the hammer mill is an envelope surface made from a bent perforated plate, corresponding to the screen tube of the former.
The screen structure of the invention is also applicable for use in classifiers used for the sorting of pulp in paper and pulp industry.
In prior art, screen plate structures are known which have been used in the above-mentioned applications, among others, in conjunction with paper production. One of the prior-art solutions is a tube with a smooth inner surface and perforations in its envelope. This solution has proved in practice to be insufficient in capacity in conjunction with faster and faster machines. Previously known are also screen tube structures the envelope of the screen tube has narrow elongated slits instead of round holes. This solution, too, is insufficient to provide a capacity that meets the present-day requirements.
To remedy the capacity problem referred to, new screen plate solutions have been developed paying attention to the cross-sectional profile of the inner surface of the screen tube, among other things. For example, Finnish patent no. FI67588, corresponding to U.S. Pat. No. 4,529,520, discloses a screen tube structure in which the inner surface of the screen tube is provided with axially extending grooves in which the upstream-side lateral surfaces are preferably substantially perpendicular to the envelope curve of the screen surface while the downstream-side lateral surfaces are inclined relative to the aforesaid plane. The bottoms of the grooves are provided with apertures or slits going through the bottom of the groove substantially at the middle of the bottom of the groove as seen in cross-section.
The above-mentioned screen tube solution and other corresponding solutions are illustrated in more detail in FIG. 1, which presents a prior-art screen tube structure comprising a screen tube bent from a plate 1a, the inner surface of the tube being provided with grooves 4a extending in the axial direction of the tube. The first lateral surface 5a of each groove 4a is at an inclined angle relative to the envelope curve 10a of the inner surface of the tube, while the second lateral surface 7a is at a substantially right angle. Correspondingly, the bottom 6a of the groove 4a is substantially parallel to the envelope curve 10a. The openings 2a going through the plate 1a are so located on the planar bottom 6a of the grooves 4a that a threshold forming corner 8a remains between the edge 3a of the opening 2a and the second lateral surface 7a of the groove 4a. A similar threshold also remains between the opening 2a and the first lateral surface 5a. 
In prior-art solutions, the fiber flow in screen tubes formed like this can be handled so that the fiber flow or a corresponding mass flow inside the screen tube is in relative motion with respect to the lower surface of the screen plate either in direction B as indicated in FIG. 1 or in the opposite direction A. In most dry formation applications, direction B, in which the trailing surface of the groove 4a, i.e. the lateral surface 7a on the upstream side is substantially perpendicular while the receiving surface, i.e. the lateral surface 5a on the downstream side is inclined relative to the envelope curve, has proved to be inferior to direction A in respect of capacity.
Correspondingly, screen structure solutions in which the fiber flow moves in direction A have the drawback that the threshold forming the corner 8a causes turbulence in the fiber flow in dry formation, such turbulence being an obstacle to smooth passage of fibers through the apertures 2a. Therefore, this solution does not provide the best possible throughput capacity, either. In other words, the main problem with this screen plate structure, too, is insufficient capacity, especially in dry formation of a web, so the former should be provided with a plurality of successive screen tube pairs to achieve a sufficient capacity.